1. Field of the Invention
The present invention relates to a touch apparatus. More particularly, the present invention relates to a readout apparatus for a touch panel.
2. Description of Related Art
With booming development of electronic technologies, and popularization of wireless communication and networks, various electronic devices gradually become indispensable tools for people's daily life. However, operation of a commonly used input/output (I/O) interface such as a keyboard or a mouse is rather difficult. Comparatively, a touch panel is an intuitive and simple I/O interface. Therefore, the touch panel is generally used as a human-machine interface between a user and an electronic device to facilitate control operations.
Generally, the touch panels are grouped into resistive touch panels, photo touch panels and capacitive touch panels, etc. Considering a readout approach, the touch panels are grouped into current type touch panels and charge type touch panels, etc. FIG. 1 is a schematic diagram illustrating a photo charge type touch panel and a conventional readout circuit. A plurality of data lines and a plurality of scan lines of the photo charge type touch panel 110 are respectively driven by a source driver 120 and a gate driver 130, and a plurality of sensor lines of the photo charge type touch panel 110 is coupled to a plurality of readout circuits 140. Only one of the scan lines, one of the data lines and one of the sensor lines are illustrated in FIG. 1.
A storage capacitor Cst1 and a liquid crystal capacitor Clc are respectively coupled to a bias voltage VBIAS1 and a common voltage VCOM. The bias voltage VBIAS1 and a common voltage VCOM can be same or difference voltage(s). When the gate driver 130 turns on a switch SW1 through the scan line, the source driver 120 writes pixel data into the storage capacitor Cst1 and the liquid crystal capacitor Clc through the data line. Since a voltage difference between the pixel data and the common voltage VCOM correspondingly deflects liquid crystal molecules in the liquid crystal capacitor Clc, the pixel may have a corresponding gray level.
Based on a bias voltage VBIAS2, a photo transistor PT provides a discharge path between a storage capacitor Cst2 and the bias voltage VBIAS2. If a position where the photo transistor PT is located is not touched by the user, the photo transistor PT accelerates discharging the storage capacitor Cst2 due to irradiation of an external light. Conversely, if the external light irradiating on the photo transistor PT is reduced due to a user's touch, the photo transistor PT slows down discharging the storage capacitor Cst2. When the gate driver 130 turns on a switch SW2 through one of the scan lines, the readout circuit 140 reads the remained charge quantity of the storage capacitor Cst2 through one of the sensor lines, and simultaneously charges the storage capacitor Cst2 to a normal rated voltage level.
When the readout circuit 140 detects the photo charge type touch panel 110, a position touched by the user is mainly determined according to inconsistent discharge of the storage capacitor Cst2 or whether there is a coupling capacitor. Regarding the photo charge type touch panel 110, an integrator (i.e. an operation amplifier 141 and a feedback capacitor Cfb) is generally configured in the readout circuit 140 to detect a charge difference of the photo charge type touch panel 110. An analog-to-digital converter (ADC) 143 converts an integration result of the integrator into a corresponding digital code, and transmits the digital code to an image processing circuit 150 for determining the touch position.
FIG. 2 is a schematic diagram illustrating a photo current type touch panel and a conventional readout circuit. A plurality of scan lines of the touch panel 210 is driven by the gate driver 130, and a plurality of sensor lines of the touch panel 210 is coupled to a readout circuit 240. A pixel layout of the conventional photo current type touch panel 210 is as that shown in FIG. 2. Each of the pixels has a switch SW1 and a photo transistor PT. When the bias voltage VBIAS is higher than a voltage of a node A and the gate driver 130 turns on the switch SW1 through the scan line, a sensing current Is will flow to the sensor line through the photo transistor PT and the switch SW1, due to the fact that the photo transistor PT is in a forward-bias state. Wherein, an intensity of the light irradiating on the photo transistor PT can influence an amount of the sensing current Is.
The readout circuit 240 is used to detect the amount and a difference of the sensing current Is on each of the sensor lines, so as to determine whether a light-shielding object is located at a corresponding position of the touch panel 210 (i.e. whether an external object touches the touch panel 210). The readout circuit 240 transmits a detection result in form of the digital code to the image processing circuit 150. The image processing circuit 150 then determines the touch position according to all of the digital codes provided by the readout circuit 240. The conventional readout circuit 240 uses an integrator (i.e. an operation amplifier 241 and a feedback capacitor Cfb) to convert the sensing current Is into a corresponding voltage, and then an ADC 243 converts the voltage into a corresponding digital code, and transmits the digital code to the image processing circuit 150 for determining the touch position.
FIG. 3 is a schematic diagram illustrating a capacitive touch panel and a conventional readout circuit. The capacitive touch panel 310 has a plurality of sensor lines in a Y-axis direction and a plurality of sensor lines in an X-axis direction. A coupling capacitor Cp is formed between each of the Y-axis direction sensor lines and each of the X-axis direction sensor lines. An integrator 320 including an operation amplifier 322 and a feedback capacitor Cfb is configured to each of the sensor lines. In the beginning, positive input terminals of all of the operation amplifiers 322 receive a 0V (volt) reference voltage Vref, and all of switches 323 are turned on, so that all of the sensor lines are charged to 0V. Thereafter, the integrators 320 turn off the switches 323 to perform readout operations. During a period when the switches 323 are turned off, assuming no conductor (for example, a finger) touches the touch panel 310, when the reference voltage Vref is transited from 0V to 5V, the integrators 320 in the Y-axis direction and the X-axis direction can equalize the voltages at two ends of the coupling capacitor Cp to 5V. Since charge/discharge of the coupling capacitors Cp is not required, when the reference voltage Vref is transited to 5V, such variation is reflected on an output of the integrator 320. After the integrators 320 complete the readout operations, all of the switches 323 are again turned on, and the above operations are repeated.
When the conductor (for example, the finger) touches the touch panel 310, an extra capacitor Cf is formed at the corresponding touch position as that shown in FIG. 3. During a period when the switches 323 are turned off, when the reference voltage Vref is transferred from 0V to 5V, the corresponding integrator 320 charges/discharges the extra capacitor Cf through the sensing line. Therefore, when the reference voltage Vref is transferred to 5V, an output OUT of the integrator 320 corresponding to the extra capacitor Cf is varied, and a formula thereof is OUT=5+[(5V−0V)×Cf]/Cfb. Then, the integrator 320 transmits the readout result to a follow-up circuit (including the ADC and the image processing circuit that are not illustrated) for determining coordinates of the touch position. According to a difference between a signal read by the sensor line having the extra capacitor Cf and a signal read by the sensor line without the extra capacitor Cf, the touch position can be determined. According to the above formula, it is known that the larger the extra capacitor Cf is, the larger the feedback capacitor Cfb is, otherwise, the output of the integrator 320 is easy to be saturated, and the touch position cannot be determined.
However, to avoid the saturation of the output of the integrator, capacitances of the feedback capacitors Cfb in the integrators has to be increased (i.e. the areas of the capacitors have to be increased). Since each of the sensor lines requires an integrator, a chip area occupied by the integrators is considerable. Moreover, different types of the touch panel require the readout circuits of different functions to read the signals of the touch panels and convert the signals into the digital signals that can be operated by the follow-up image processing circuits. For example, a capacitance of the feedback capacitor Cfb of FIG. 3 is much bigger than a capacitance of the feedback capacitor Cfb, so that the readout circuit 140 applied to the photo charge type touch panel 110 cannot be applied to the capacitive touch panel 310. If each type of the touch panel applies the readout circuit of a different function, a usage flexibility of the readout apparatus of the touch panel is relatively low.